1. Field of the Invention
The present invention relates to an output control apparatus of a generator, and particularly to an output control apparatus of a generator suitable for further improvement of output voltage waveform by maintaining an electric current flowing a field winding constant.
2. Description of the Related Art
In a power generation control apparatus of an alternating-current (AC) generator which includes a power generation winding, excitation winding, and field winding, and rectifies electric current generated in the excitation winding and supplies the same to the field winding, automatic voltage regulator is known in the art which maintains a voltage output from the power generation winding to a predetermined voltage by controlling the electric current supplied to the excitation winding.
FIG. 6 is a drawing showing composition of primary components of the AC generator equipped with conventional automatic voltage regulator. In FIG. 6, a generator 100 includes a field winding 102, a power generation winding 103, and an excitation winding 104. A permanent magnet 106 is provided to a rotor 105 around which the field winding 102 is wound. An automatic voltage regulator (AVR) 107 are provided with a rectifier 108, a voltage detection circuit 109, and a transistor 110. Output of the rectifier 108 is connected to the field winding 102 via a brush 111, and a flywheel diode 112 and a capacitor 113 are connected in parallel with the field winding 102. The excitation winding 104 is connected to input side of the rectifier 108. The rotor 105 is driven by a driving source such as engine (not shown).
In FIG. 6, when the rotor 105 is driven by the engine or the like, an electric current is generated to the excitation winding 104 by the permanent magnet 106. This electric current is rectified by the rectifier 108 and is supplied to the field winding 102 in the form of direct-current (DC) excitation current. The voltage detection circuit 109 compares an output voltage of the power generation winding 103 with a predetermined voltage and executes ON/OFF control of the transistor 110 depending on magnitude of the output voltage with regard to the reference voltage. When the transistor 110 is ON, electric current flows through the field winding 102 thereby increasing output of the power generation winding 103, and when the transistor 110 is OFF, electric current does not flow through the field winding 102 and output of the power generation winding 103 is decreased. Output of the generator 100 is maintained constant by ON/OFF control of the transistor. An example of such AC generator equipped with the automatic voltage generator is described in Japanese Patent Application Laid-Open (JP-A) No. 08-140400.
In the above-mentioned generator, by flowing an electric current through the field winding 102, an induced current is generated to the power generation winding 103, then a back electromotive force is induced to the field winding 102 by magnetic field generated by the electric current. Since the electric current flowing through the field winding 102 is increased or decreased by the back electromotive force, output of the power generation winding 103 fluctuates. However, with conventional automatic voltage regulator 107 which is based on the principle that current increase in constant direction is stopped by the transistor 110, electric current flowing through the flywheel diode 112 connected in parallel with the field winding 102 can not be decreased. When field current is PWM controlled based on voltage detection in the voltage detection circuit 109, the flywheel diode 112 is indispensable to absorption of surge voltage generated at energizing stop and smoothing of the field current.
Therefore, with the control apparatus having the flywheel diode 112, it is not easy to maintain the electric current flowing through the field winding 102 constant. When the electric current flowing through the field winding 102 can not be maintained constant, output voltage of the power generation winding 103 has distortions with respect to the sinusoidal wave. Hence, further improvement of the output voltage waveform is desired.